let navLiList = document.getElementById("nav_left").children[1].children;
let contentLiList = document.getElementById("content_right").children;
for (let i = 0;i< navLiList.length; i++){
        navLiList[i].onclick = function(){
            navLiList[i].setAttribute('index',i);
            let index = navLiList[i].getAttribute('index');
            for (let j = 0; j<contentLiList.length; j++) {
                contentLiList[j].style.display="none";
            } 
            contentLiList[index].style.display="block";
        }
} 

window.onload = function(){
	document.getElementById("manage1").style.display = "block";
}